1. Field of the Invention
The present invention relates in general to a micro-electro mechanical systems (MEMS) device using a silicon-on-insulator (SOI) wafer, and more particularly to an MEMS device using an SOI wafer in which a handle wafer of the SOI wafer is connected to a ground hole without any additional bonding or wiring process, and a method for fabricating and grounding the same.
2. Description of the Related Art
An MEMS device is a product implemented by combining micro-mechanical components such as micro-sensors, micro-valves, micro-gears, micro-mirrors and micro-actuators formed in a semiconductor chip and a computer technology, and thus called an “intelligent product”. An MEMS device may further include microcircuits in the semiconductor chip having the micro-mechanical components thereon. Such MEMS devices may be produced cost effectively for a wide variety of applications because they can be fabricated with at low cost in large volumes. In such a MEMS device, if a handle wafer (base wafer) of an SOI wafer in the MEMS device is electrically floated, parasitic capacitance between the handle wafer and a structure formed on the SOI wafer for acting as a micro-mechanical component causes electrical interference that may negatively influence the micro-mechanical component such as an actuator or a sensor. Accordingly, the handle wafer should be electrically connected to a ground area for preventing the parasitic capacitance.
FIG. 1 illustrates a sectional view of a typical SOI wafer 10. Referring to FIG. 1, the SOI wafer comprises a handle wafer 11 serving as a base wafer, a device wafer 13 provided over the handle wafer 11 and a sacrificial layer 12 formed of an insulation layer and interposed between the handle wafer 11 and the device wafer 13.
The handle wafer 11 has a thickness of several hundreds of μms and serves as a substrate in general semiconductor devices. The device wafer 13 has a thickness from several tens to several hundreds of μms and the thickness of the device wafer 13 may vary based on products. The device wafer 13 may has a desired structure by being etched. The sacrificial layer 12 is made of silicon oxide. In the case that the desired structure is a gyroscope or accelerometer, the sacrificial layer 12 may be removed for providing a vacant space under the desired structure so that the desired structure is allowed to move in the MEMS device.
FIG. 2 illustrates a sectional view of a conventional wire bonding type MEMS device. Referring to FIG. 2, the wire bonding type MEMS device 20 comprises a first silicon layer 21 corresponding to a handle wafer, an insulation layer 22 formed of silicon oxide, a second silicon layer 23 corresponding to a device wafer, and a protective layer 24 formed of glass material. The protective layer 24 has a signal hole SH and a ground hole GH therein. The signal hole SH and the ground hole are filled with a conductive material layer CM. A metal pad MP is formed on the upper surface of the conductive material layer CM in the ground hole GH.
The handle wafer 21 is attached to a grounded surface of a printed circuit board PCB by a conductive adhesive. The metal pad MP formed on the conductive metal layer CM in the ground hole GH is connected to a ground area GA on the PCB by a wire bonding method. The MEMS device 20 is encapsulated with a resin such as epoxy by transfer molding.
The wire bonding type MEMS device described above has a drawback that a wire bonding process is necessarily required to connect the metal pad in the ground hole GH to the ground area GA of the PCB, thereby causing inconvenience. The wire bonding type MEMS device in accordance with the conventional art is further disadvantageous in that contact reliability is degraded, even in the case that the conductive adhesive is used, because a natural oxide layer is formed on a silicon surface even when the silicon surface is exposed to the air for a certain period of time.
On the other hand, as shown in FIG. 3, a gold flip chip bonding type MEMS device has been recently developed and it is advantageous in terms of miniaturization and noise reduction. Details of the gold flip chip bonding type MEMS device will be described below with reference to FIG. 3.
FIG. 3 is a sectional view of a conventional gold flip chip bonding type MEMS device 30. The gold flip chip bonding type MEMS device is packaged by using a gold bump flip chip bonding method which is a sort of flip chip bonding method. The basic structure of the gold flip chip bonding type MEMS device 30 is the same as the wire bonding type MEMS device 20 shown in FIG. 2. However, the gold flip chip bonding type MEMS device 30 is different from the wire bonding type MEMS 20 in terms of the mounting direction to the PCB.
As shown in FIG. 3, the gold bump flip chip MEMS device 30 comprises a first silicon layer 31 corresponding to a handle wafer, an insulation layer 32 formed of silicon oxide, a second silicon layer 33 corresponding to a device wafer, and a protective layer 34 made of glass material. The protective layer 34 has a signal hole SH and a ground hole GH, both of which are filled with a conductive material layer CM. The conductive materials CM in the signal hole and the ground hole are connected to respective metal pads MP. The respective metal pads MP in the signal hole SH and the ground hole GH are connected to a signal and ground areas on a printed circuit board PCB with respective gold balls GB.
For the gold bump flip chip bonding, a gold ball is placed between the respective metal pads MP on the MEMS device 30 and the associated signal or ground area on the PCB, and ultrasonic vibration and pressure are applied to the MEMS device 30 while the MEMS device 30 maintains a desired temperature. Such gold flip chip bonding type MEMS device is advantageous in terms of miniaturization and noise reduction characteristics.
The gold flip chip bonding type MEMS device, however, is disadvantageous in that it is difficult to connect an upper substrate (handle wafer) of the MEMS device to a printed circuit board. Accordingly, there are grounding problems even in the flip chip bonding method that is a new technology attracting a great deal of attention.